Typical voice-type paging systems have employed analog voice channels for the transmission of voice messages. While certain types of paging systems utilize binary signalling formats, transmission in an analog form remains the most common technique for voice signals. Paging systems that transmit analog representations of voice signals are limited in the number of paging subscribers that can be supported by one R.F. channel. In particular, due to the length of a voice message, which may range from five to over twenty seconds, only 1,200 to 1,500 customers can be supported on a typical tone and voice paging channel, while as many as 100,000 users can be supported on a channel that transmit only address signals. The limited number of users that can be placed on a voice channel, together with the limited number of channels available for paging, have several effects. First, the cost per user of the channel must be fairly high to support the cost of running the system. Second, until the release of 900 Mhz spectrum, there were not enough channels available to support the demand for tone and voice paging. Third, the lack of spectrum adversely affected the sales of new paging equipment.
In addition, the traditional analog signalling format does not provide several features that would be highly desirable. These include the ability to store a voice message in a reasonable size memory for recall at a later time, and the use of a digital modulation format to ease system problems. The use of Linear Predictive Coding (LPC) encoding of the speech at the terminal end of the paging system, and the subsequent storage and reconstruction of voice messages in paging receivers offers several improvements over present systems. First, present LPC technology allows good voice quality to be achieved at a data rate of 2400 bits per second. Using this voice data rate in conjunction with the 1200 bits per second digital modulation that can be used on paging transmitters increases the message throughput of a paging channel by a factor of five. Thus, the maximum number of users than can be placed on a tone and voice channel can be increased by a factor of five to 6,000 to 7,500. Furthermore, using LPC encoded speech data at 2400 bits per second permits speech messages to be stored in a reasonable size memory. That is, 26.667 seconds of speech can be stored in a 64K bit RAM for a 2400 bit per second data rate.